Based on String theory, there is a 3-cosmic framework of the Universe, which has triple cosmoses in the whole of space that can find a dark planet, which is about 1.33 times the mass of Mars, located inside the Earth, but in the other cosmos than ours. Based on the data of the cosmological parameters from the first year of WMAP observations to the Planck satellite observations in 2018, it can be speculated that the current dark energy should be taken as the residual energy of the Universe today after the Big Ban. Due to the rapid expansion of other high-energy-density cosmoses, its dark matter should exert a gravitational pull on the stars of our low-energy-density cosmos that causes the effect of accelerating the expansion of our cosmos.